The present invention relates to solid state dye lasers and especially to a gain medium for solid state dye lasers comprising at least one cholesteric liquid crystalline polymer layer which provides distributed feedback for laser light emitted in the gain medium.
Laser devices are used in all areas of science and technology to generate coherent electromagnetic radiation in the infrared, visible or ultraviolet region the spectrum, i.e. approximately at wavelengths between 300 and 1800 nm. Lasers operate using the principle of light amplification by stimulated emission of radiation. If light is incident on an excited species, they may be stimulated to emit their energy as additional light with the same frequency, phase, polarisation and direction as the incident light. As the converse process, i.e. stimulated absorption of unexcited species, is also possible in a given sample, net amplification only occurs when the excited species outnumber the unexcited species, i.e. when an “inversion” of the atomic levels is established. Emission of laser light requires an excitable active species, e.g. a dye, which exhibits strong stimulated emission of radiation. Further, optical feedback means must be provided which act as a resonator for laser light emitted by the active species. The simplest resonator consists of two opposing mirrors arranged on opposite sides of a medium containing the active species which is conventionally denoted “gain medium”.
Lasers are conventionally classified according to the type of gain medium used, e.g. gases, dyes, solid state semi conductor, and others.
Among those, dye lasers are of particular interest in many technical fields because they provide a broad tuneability of the emitted laser light over the spectral range mentioned above and their pumping methods, i.e. the methods for exciting the active species, are rather flexible. Commonly, excitation of the dye is achieved by means of so-called optical pumping using a source of energy such as a flash lamp or a pump laser. Typical pump lasers are nitrogen, argon iron, and frequency doubled Nd:YAG (neodymium/yttrium-aluminum-garnet). Dye lasers can be operated in either continuous-wave (CW) mode with continuous powers of typically up to 100 mW with very narrow line width or in pulsed mode with energies up to 1 J and pulse durations in the femto seconds range.
While most dye lasers operate with a liquid gain medium, solid-state dye lasers have also been developed, a laser assembly where the laser dyes are incorporated e.g. in a solid polymer matrix such as polymethyl methacrylate (PMMA). Solid state gain media overcome some of the disadvantages of liquid gain media such as handling problems and health or environmental hazards associated with many laser dyes and solvents commonly employed.
As an alternative to using end mirrors to define an optical cavity, mirrorless dye lasers with optical feedback distributed throughout the gain medium are known. Distributed feedback is commonly used in semiconductor or dye lasers, especially, when single mode operation is required (e.g. Shank et al. “Tunable distributed-feedback dye laser”, Applied Physics Letters, 18, 152 (1971)). The distributed feedback is obtained by a gain medium exhibiting a spatial modulation in optical properties such as refractive index or gain in the direction of light propagation through the medium. A common method to obtain a periodic modulation in the gain medium is to interfere two coherent pump laser beams. Then, the output wave length of stimulated laser light is proportional to the periodicity of the interference pattern. The laser emission wavelength can be tuned e.g. by varying the angle between the pump laser beams.
In U.S. Pat. No. 3,771,065, a liquid gain medium for dye lasers has been suggested consisting of a laser dye dissolved in a cholesteric liquid crystal (CLC) which provides distributed feedback. Such gain media benefit from special optical properties of the cholesteric or “chiral nematic” phase of certain liquid crystals: CLC's develop a helical superstructure characterized by a local nematic director which is perpendicular to the helix axis but varies linearly with its position along the helix axis. The pitch, i.e. the spatial period, is determined by the concentration and the helical twisting power of the chiral constituents. As a consequence of the periodicity of the helical cholesteric structure and the birefringence of the liquid crystal, for a range of wavelengths, light propagation along the helix axis is forbidden for one of the normal modes. Thus incident light of a “forbidden” wavelength is strongly reflected. The edges of this relection band are at wavelengths which are equal to the refractive indices times the helical pitch (c. f. deGennes, “The physics of liquid crystals”, Clarendon Press, Oxford, 1974). Thus, if a dye doped CLC is aligned between two glass plates in the so-called planar texture, a “Bragg-type” phase grating is established throughout the CLC layer. Then laser emission is normal to the film plane and the output wavelength is set by the helical periodicity. By varying the temperature of the CLC host, the helical pitch of the CLC host can be changed, thereby the output wavelength of the dye laser can be tuned. Fluid CLC gain media are, however, subject to environmental perturbation, such as temperature, and impractical for many applications.
In U.S. Pat. No. 6,141,367, the disclosure of which is hereby incorporated by reference into the present application, a solid state dye laser has recently been described which has a solid gain medium doped with a fluorescent dye. It has been suggested to use a gain medium which includes a polymeric cholesteric liquid crystal disposed in a planar texture and frozen into a characteristic wavelength. The configuration of the dye laser including location means which allow for locating and orienting the gain medium relative to a pump laser are extensively described in this document. However, U.S. Pat. No. 6,141,367 does not disclose any specific polymeric CLC which can act as a suitable host material of a solid state dye laser medium.
Lasing in dye doped cholesteric liquid crystals has been demonstrated for the first time by Kopp et al. in Opt.Lett. 23, 1709, 1998 and Taheri et al. in ALCOM Symposium on Chirality, Cuyahoga Falls, February 1999. Subsequently, Finkelmann et al. have suggested in Adv. Mater. 2001, 13, No. 14, 1069-1072 to use an elastomeric cholesteric liquid crystal as a laser gain medium. Finkelmann et al. have demonstrated that mechanical stretching of an elastomeric cholesteric liquid crystal allows for tuning the lasing wavelenth. The elastomeric liquid crystal used by Finkelmann et al. comprises a polymeric network synthesized via a hydrosilylation of poly[oxy(methylsilylene] both with an achiral nematogenic monomer, said monomer having a mesogenic group comprising
a chiral cholesterylcarbonate and 1,3,5 triallyloxybenzene as a crosslinking agent. As laser properties such as lasing threshold, line width, pulse duration etc. are strongly influenced by the optical properties of the distributed feedback gain medium, there is a need for polymer CLC's which prove particularly suitable for use in a solid state dye laser.